The problem is that your radio does not distinguish between 2 meters and 70 cm with that relay for activating the amplifier (if it uses relay activation). Or, if the amplifier activation is by RF detection, then the RF detection circuit can't distinguish between 2 meter RF and 70 cm RF. 'Curing' that problem ought to have several solutions, but I'm not aware of a commercially available device that would do that, are you? - Paul

I am using RF sensing for the amplifier. The one amplifier I used would switch on with 70cm RF.

I have a radio with separate 2m and 70cm outputs. I may go this route so I could use an amplifier on the 2m side but then I suppose I would need two antennas.

I may do this in the house also but for now I am setting this up mobile. If anyone has any pictures or ideas of a 2 antenna crossband setup and/or a crossband setup with an amplifier I'd be interested.

I did this several years ago when my club's 2M repeater was on the fritz. My radio was the "transmit" side of the temporary repeater. My buddy's radio about 2 miles away was the "receive" side of the temporary repeater. We linked the two radios via 70CM.

I fed my Kenwood D7A into a diplexer to split off V from U, ran the V side into a Mirage 2M brick then combined the V and U again with a second diplexer to a single feedline to a dual-band base antenna. The setup worked reasonably well and the horizontal distance between the two radios was enough to prevent desense issues.

Since your radio already has separate V and U outputs, you'll only need to combine them again using a diplexer after the 2M amp to feed a single antenna (if you want to avoid having to install two antennas).

A duplexer is not the same as a diplexer. A duplexer is a cavity filter that is tuned to notch a certain frequency. They're frequently used in in-band repeater installations.

A diplexer splits or combines signals from separate frequency ranges. Comet, Diamond, etc. make them for either splitting a multi-band antenna to feed separate, monoband radios or for feeding a radio with multiple outputs into one antenna.

Check to see if the output of your 2M amp adjusts depending on the input power. You may be able to feed it with, say, 10 watts and have it dial back the output to 100 watts.

A duplexer is not the same as a diplexer. A duplexer is a cavity filter that is tuned to notch a certain frequency. They're frequently used in in-band repeater installations.

A diplexer splits or combines signals from separate frequency ranges. Comet, Diamond, etc. make them for either splitting a multi-band antenna to feed separate, monoband radios or for feeding a radio with multiple outputs into one antenna.

Those are pretty common definitions, I think, but not supported by industry.

Duplexer was intended to mean a device allowing duplex operation, i.e., transmitting and receiving with a single antenna. Doesn't matter what that device is, or how widely separated the frequencies are.

Diplexer was intended to mean a device allowing multiple transmitters to share a common antenna, without regard for duplex operation. Doesn't matter what that device is, or how widely separated the frequencies are.

For example, WCBS-AM and WNBC-AM BC radio stations in NYC share a common antenna. They operate at 880 kHz and 660 kHz respectively (in the same band, within 220 kHz of each other). The device used to allow that is called a "diplexer." I'm pretty familiar with that one, since I've worked on it.

Yeah I could run the amplifier at 100 watts possibly, but I don't want to go through the trouble for a small gain. I think I'd rather go with two antennas. Not sure if it's worth the hassle putting an amp in or not if I want to be able to crossband repeat.

Before, when I had an amp in there, I just wouldn't use it for crossband repeating.

If your radio has two separate band connections for antennas, use two antennas. It's by far the simplest and the safest--for the sake of the radio--way to do what you're looking to do. Even the hassle of antenna separation is minor for VHF/UHF operation compared to the task of making sure everything is set up correctly and then testing it if using a single antenna.

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